Up to now, a device configured as shown in FIG. 1 is used as an optical pickup device for reading out information signals recorded on an optical disc, such as a magneto-optical disc.
This optical pickup device includes a semiconductor laser 2 for radiating a light beam L.sub.1 illuminated on an optical disc 1 and an objective lens 3 for converging the light beam L.sub.1 outgoing from the semiconductor laser 2 for illumination on the optical disc 1, as shown in FIG. 1.
On the light path of the light beam L.sub.1 from the semiconductor laser 22 to the objective lens 3 is a grating 4 disposed towards the semiconductor laser 2 for splitting the light beam L.sub.1 outgoing from the semiconductor laser 2 into at least three diffracted light beams, namely a zero-order light beam and .+-.1 order light beams, as shown in FIGS. 1 and 2.
It is noted that, in FIGS. 1 and 2, the three diffracted light beams are represented as a sole light beam and these three diffracted light beams are referred to simply as a light beam in keeping with the drawing.
On the light path from the grating 4 to the objective lens 3 are disposed a beam splitter 5 and a reflective mirror 6. The beam splitter 5 splits the light beam L.sub.1 outgoing from the semiconductor laser 2 from a light beam L.sub.2 reflected by the optical disc 1 to fall on the beam splitter 5 via objective lens 3, while the reflective mirror 6 causes the light beam L.sub.1 transmitted through the beam splitter 5 to be deflected 90.degree. to fall on the objective lens 3 while causing the light beam L.sub.2 reflected by the optical disc 1 and transmitted through the objective lens 3 to be deflected 90.degree. to fall on the beam splitter 5.
The optical pickup device includes a photodetector 7 for detecting the return light beam L.sub.2 reflected back from the optical disc 1 to read out the information signals recorded on the disc and for detecting focusing error signals and tracking error signals. The photodetector is comprised of a light detecting element, such as a photodiode. This photodetector 7 is arranged facing the beam splitter 5 at a position perpendicular to the light path from the semiconductor laser 2 to the objective lens 3 for detecting the return light beam L.sub.2 the light path of which has been modified 90.degree. by being reflected back by a boundary surface 5a of the beam splitter 5. Meanwhile, the boundary surface 5a of the beam splitter 5 is inclined 45.degree. relative to the optical axis of the light beam incident on the beam splitter 5 for modifying the light path of the light beam incident on the beam splitter 5 by 90.degree..
In the light path from the beam splitter 5 to the photodetector 7 are arranged a Wollaston prism 8 for detecting the Kerr rotation angle of the return light beam L.sub.2 reflected by the optical disc 1 and a cylindrical lens 9 for producing astigmatic aberration in the return light beam L.sub.2. The Wollaston prism 8 is mounted as-one with the beam splitter 5.
The present optical pickup device has a photodiode 10 for detecting part of the light beam L.sub.1 radiated from the front side of the semiconductor laser 2 for controlling the semiconductor laser 2 to render the output level of the light beam radiated from the semiconductor laser 2 substantially constant. This photodiode 10 is arranged facing the beam splitter 5 at a position perpendicular to the light path from the semiconductor laser 2 to the objective lens 3 for detecting a portion of the light beam L.sub.1 from the semiconductor laser 2 having its light path changed 90.degree. by being reflected back by a boundary surface 5a of the beam splitter 5.
The detection output of the photodiode 10 is supplied to an automatic output control circuit 12 configured for controlling the intensity of the driving current supplied from a driving source 11 configured for driving the semiconductor laser 2. The automatic output control circuit 12 is responsive to a detection output of the photodiode 10 to control the driving current supplied from the driving source 11 so that the semiconductor laser 2 will radiate the light beam L.sub.1 of a constant output level.
Meanwhile, since the return light beam L.sub.2 incident on the photodetector 7 and a portion L.sub.3 of the light beam L.sub.1 radiated from the semiconductor laser 2 to fall on the photodiode 10 are changed in light paths by being reflected by the common boundary surface 5a of the sole beam splitter 5, the photodetector 7 and the photodiode 10 are arranged facing each other on both sides of the beam splitter 5 as shown in FIG. 2.
In the above-described optical pickup device, if the driving current is supplied from the driving source 11 for driving the semiconductor laser 2, the light beam L.sub.1 is radiated from the semiconductor laser 2. The light beam L.sub.1 radiated from the front side of the semiconductor laser 2 is split by the grating 4 into at least three diffracted light beams to fall on the beam splitter 5. The portion L.sub.3 of the light beam L.sub.1 from the semiconductor laser 2, having its light path changed 90.degree. by the boundary surface 5a of the beam splitter 5, is received by the photodiode 10 whereby the intensity of the portion L.sub.3 of the light beam 1 is converted to an electrical signal which is detected. This detection output is supplied to an output control circuit 12 for controlling the driving current supplied from the driving source 11 to the semiconductor laser 2 for controlling the driving of the semiconductor laser 2 for providing a constant output level of the light beam L.sub.1 radiated from the semiconductor laser 2.
The light beam L.sub.1, transmitted through the beam splitter 5 without being reflected by the boundary surface 5a of the beam splitter 5, falls on the reflective mirror 6 so as to be thereby changed in light path by 90.degree. to then fall on the objective lens 3. The light beam L.sub.1, reflected by the reflective mirror 6, is converged by the objective lens 3 on the signal recording surface of the optical disc 1. The light beam L.sub.2, reflected by the signal recording surface of the optical disc 1, again falls via objective lens 3 on the reflective mirror 6 so as to be thereby re-converted in light path by 90.degree.. The light beam L.sub.2, the light path of which has been changed by 90.degree. by the reflective mirror 6, falls on the beam splitter 5 to be then reflected by 90.degree. by the boundary surface 5a. The light beam L.sub.2, reflected by 90.degree. by the reflective mirror 6, falls on the Wollaston prism 8 for detecting the Kerr effect so as to be then received via cylindrical lens 9 by the photodetector 7.
Due to the cylindrical lens 9, the light beam L.sub.2 reflected by 90.degree. by the boundary surface 5a undergoes astigmatic aberration depending on changes in the distance between the objective lens 3 and the signal recording surface of the optical disc 1. The result is that the spot shape is changed on the light receiving surface of the photodetector 7 depending on changes in the distance between the signal recording surface of the optical disc 1 and the objective lens 3, so that focusing error signals are produced based on an output signal of the photodetector 7. On the other hand, each spot corresponding to the .+-.one order diffracted light on the light receiving surface of the photodetector 7 is moved depending on how much the light spot of each of the .+-.one order diffracted light beams on both sides of the spot of the zero order diffracted light beam radiated on the signal recording surface of the optical disc 1 is shifted relative to the recording track of the optical disc 1. The result is that the output signal of the photodetector 7 is changed so that the tracking error signals can be generated based on the changes in the output signal of the photodetector 7. Moreover, the information signals recorded on the optical disc 1 can be read out based on the output signal of the photodetector 7 receiving the zero-order diffracted light.
Since the above-described optical pickup device controls the light beam L.sub.1 outgoing from the semiconductor laser 2 to a constant output level, the photodetector 10 for detecting part of the light beam L.sub.1 radiated from the semiconductor laser 2 is arranged towards the light path of the light beam L.sub.1 proceeding from the semiconductor laser 2 to the objective lens 3. Since the photodetector 10 is configured for detecting part of the light beam L.sub.1 reflected by the boundary surface 5a of the beam splitter 5, the photodetector 10 needs to be provided at a position at right angles to the light path of the light beam L.sub.1 proceeding from the semiconductor laser 2 to the objective lens 3. Moreover, the photodetector 10 needs to be arranged facing the photodetector 7, adapted for detecting the return light beam L.sub.2 reflected from the optical disc 1, with the beam splitter 5 in-between.
If the photodetector 10 is arranged in this manner, the direction perpendicular to the light path from the semiconductor laser 2 to the objective lens 3 is increased in width, thus excessively increasing the size of the optical pickup device itself. If the optical pickup device is increased in size, the optical disc recording and/or reproducing apparatus is also increased in size.
Thus, an optical pickup device shown in FIG. 3 has so far been proposed, which is reduced in size for enabling size reduction of the disc recording and/or reproducing apparatus employing the optical pickup device and which can be controlled for prohibiting fluctuations in the output level of the outgoing light beam for assuring a substantially constant output level of the light beam.
The optical pickup device shown in FIG. 3 has, in a casing 14 housing the semiconductor laser 2, a photodiode 15 for receiving the light beam radiated towards the back side of the semiconductor laser 2. This photodiode 15 detects the intensity of the light beam radiated by the photodiode 10 towards the back surface of the semiconductor laser 2. This detection output is sent to an automatic control circuit 12 configured for controlling the intensity of the driving current supplied from the driving source 11 for driving the semiconductor laser 2. This automatic control circuit 12 is responsive to a detection output detected by the photodiode 10 for controlling the intensity of the driving current supplied from the driving source 11. The semiconductor laser 2 is driven by the driving current, the intensity of which is controlled depending on the intensity of the light beam radiated from the semiconductor laser 2, for radiating the light beam L.sub.1 of a perpetually constant output level.
By providing the photodiode 15 for detecting the light beam radiated towards the back side of the semiconductor laser 2, it becomes unnecessary to provide the photodiode 10 for detecting part of the light beam L.sub.1 radiated towards the front side of the semiconductor laser 2, so that the direction perpendicular to the light path from the semiconductor laser 2 to the objective lens 3 can be reduced in width to render it possible to reduce the size of the optical pickup device itself and the optical disc recording and/or reproducing apparatus employing the optical pickup device.
However, part of the light beam L.sub.1 radiated from the semiconductor laser 2 to fall on the beam splitter 5 is reflected by the boundary surface 5a of the beam splitter 5. The portion L.sub.3 of the light beam L.sub.1 reflected by the boundary surface 5a has its light path changed by 90.degree. to proceed towards a base member 16 carrying the beam splitter 5 or the grating 4. The base member 16 is formed of metal, such as aluminum, and hence is high in reflectivity. Thus, the light beam L.sub.3 directed to the base member 16 is reflected by its surface to return again towards the beam splitter 5. The light beam L.sub.3 reflected towards the beam splitter 5 proves to be a stray light component which is passed through the beam splitter 5 to fall on the photodetector 7. If the stray light, which is the light beam L.sub.3 other than the return light beam L.sub.2 reflected by the optical disc 1, falls on the photodetector 7, it becomes impossible to detect the return light beam L.sub.2 correctly by the photodetector 7.
If the return light beam L.sub.2 cannot be detected correctly, it becomes impossible to detect the focusing error signals or tracking error signals correctly, such that it becomes impossible to perform focusing control of displacing the objective lens 3 along the optical axis for tracking control of displacing the objective lens 3 in the planar direction perpendicular to the direction of the optical axis of the objective lens 3. Thus, it becomes impossible to correctly scan the recording track formed on the optical disc 1 by the light beam L.sub.1 converged by the objective lens 3 to be illuminated on the signal recording surface of the optical disc 1, such that it becomes impossible to read out the information signals correctly.
On the other hand, part of the light beam L.sub.3 reflected by the surface of the base member 16 to fall again on the beam splitter 5 is reflected by the boundary surface 5a of the beam splitter 5 so as to be deflected by 90.degree. in its light path and directed towards the semiconductor laser 2. The light beam L.sub.3 returned towards the semiconductor laser 2 interacts with the light beam L.sub.1 outgoing from the semiconductor laser 2 to cause resonance to generate so-called scoop noise. This renders the output level of the light beam L.sub.1 outgoing from the semiconductor laser 2 and the output level of the return light beam L.sub.2 detected by the photodetector 7 unstable. Consequently, the focusing error signals or tracking error signals cannot be detected correctly by the photodetector 7, such that information signals cannot be read out correctly.